The present invention relates generally to circuits useful in the operation of high intensity discharge lamps, and more particularly to ignition circuits for high intensity discharge lamps.
High intensity discharge ("HID") lamps such as, for example, metal halide, high pressure sodium, high pressure mercury, and metal vapor require ignition before they are able to operate in their "arc" stages and furnish their rated illumination. Ignition of an HID lamp requires the application of a high voltage pulse, typically a few thousand volts, across the terminals of the lamp. Ignition and lamp operation is performed by circuits known as "ballasts."
FIG. 1 shows a ballast circuit 10 which is useful for igniting and operating an HID lamp 38. Direct current ("DC") voltage is generated by DC source 12, suitable designs for which are well known in the art. The voltage from the DC source 12 is supplied to a down converter 14, which functions as a current source with reduced voltage relative to the output of the DC source 12. Suitable designs for the down converter 14 are well known in the art. The output of the down converter 14 is supplied to a commutator 16, suitable designs for which are well known in the art. The output of the commutator 16 applies a periodically reversing current flow to a secondary winding 34 of a transformer 30 and the lamp 38, which are connected in series. Power is furnished to the lamp 38 over cable 36, the length of which typically ranges from a foot or so to fifteen feet.
Ignition of the lamp 38 is handled by the ignition circuit 20. In addition to the secondary winding 34, the ignition circuit 20 includes inductor 22, the primary winding 32 of the transformer 30, two SIDACs 24 and 26, and the parallel combination of resistor 28 and capacitor 29, all connected in series between the output of the down converter 14.
The ignition circuit 20 operates as follows to ignite the lamp 38. A capacitor (not shown) at the output of the down converter 14 charges based on the switching frequency and duty cycle of a transistor switch (not shown) in the down converter 14. The voltage across the SIDACs 24 and 26 is equal to the voltage across the capacitor at the output of the down converter 14 until the breakover of the SIDACs 24 and 26 occurs, at which time a voltage pulse is applied to the primary winding 32 and coupled to the secondary winding 34 as a high voltage pulse. To ensure good coupling between the primary winding 32 and the secondary winding 34 so that a good ignition pulse is achieved, the core of the transformer 30 is ungapped. The SIDACs 24 and 26 remain ON until the current through them falls below their holding current, when they turn OFF. At this time, capacitor 29 discharges through the resistor 28. Now, if the lamp 38 has ignited, the down converter 14 provides a large current to the lamp 38 through the commutator 16, but the output voltage of the down converter 14 drops below the breakover voltage of the SIDACs 24 and 26 so that the ignition circuit 20 becomes inactive. On the other hand, if the lamp 38 does not ignite, the voltage across the capacitor at the output of the down converter 14 begins to increase until it becomes equal to breakover voltage of the SIDACs 24 and 26 and the ignition cycle repeats. The inductor 22 limits di/dt to protect the SIDACs 24 and 26.
Suitable values for the various components of the ignition circuit 20 designed for driving a 100W ceramic metal halide lamp, for example, are as follows: inductor 22, 47 .mu.H; SIDAC 24, type MKP1V120 or equivalent; SIDAC 26, type MKP1V120 or equivalent; resistor 28, 10K.OMEGA.; and capacitor 29, 220 nF. The transformer 30 is of the ungapped type having a E25/13/11 bobbin with four sections, a 3C85 ferrite core, a wire primary of 9 turns of 0.45 wire, and a wire secondary of 132 turns of 0.45 wire.